The present invention relates to a discriminating apparatus for television signal including scrambled video signal (scrambled television signal) and, more particularly, to such apparatus adapted for use with a wireless pay television system in which the television signal scrambled by an encoder of the broadcasting station offering a charged sustaining program is reproduced into proper picture and sound in the television receiver of a specified subscriber by means of a decoder attached thereto, said apparatus automatically discriminating between normal television signal and the television signal from said system.
The so-called pay television system was developed in accordance with demands for more substantial and enriched programs even with charges in view of tiresome commercial messages unavoidable in charge-free commercial broadcasting and decline in quality of programs due to various limitations. The wireless pay television system thus developed has advantages such that cost and time for laying cables can be eliminated and theoretically there is no limitation in the number of subscribers to a system unlike cable television system since the system is wireless. Because of these advantages, the wireless pay television system is regarded to have a noticeable future.
Generally, in the wireless pay television system intended exclusively for specified subscribers, video and sound signals are encoded to exclude non-subscribers from receiving the programs.
While the pay television system is intended exclusively for the specified subscribers as mentioned above, it is desirable that the existing ordinary television set can be used commonly for the pay television system also. Accordingly, in such system, a decoder which decodes the encoded signal is prepared for the specified subscribers, by only such decoder being added to the existing ordinary television set, either signal from general television broadcasting and pay television broadcasting is selective. While the television signal of many general television broadcasting is modulated in negative modulation system, one of pay television broadcasting is in positive modulation, the decoder must discriminate automatically to which broadcasting the received signal belongs. This is true also in the case where a mode of modulation is reversed.
FIG. 1 shows a block diagram of an embodiment of the discriminating apparatus for television signal according to the present invention, which includes a novel control means 100 added to a conventional discriminator for encoded television signal. A tuner 1 selects the desirable electric wave out of the received waves and converts it into the IF frequency signal. An IF amplifier and demodulator 2 amplifies and demodulates the output from the tuner 1, the demodulated video signal (television signal) is supplied to a switching circuit 9 through invertors 3 and 4, to a first sync separator through an invertor 5, to a second sync separator through a buffer 6, respectively. When the pay television system adopts the positive modulation system, the received television signal is as shown in FIG. 2(A), on the other hand, in the normal television system adopting the negative modulation system, the received television signal is as shown in FIG. 2(B). Accordingly, after the received signal is demodulated, either envelope curve A1 or B1 in accordance with the mode of modulation of the broadcasting station is supplied to the invertors 3 and 5, the buffers 4 and 6.
On the assumption that the received signal is transmitted from the pay television broadcasting station, the signal applied to the first sync separator 7 through the invertor 5 is as the curve A2 (disregarding DC level) inverting the curve A1. Similarly, the signal applied to the second sync separator 8 is as the curve A1, but it is the same as the curve B2 disregarding DC level. The first and second sync separator function similarly to the conventional sync separator, if the threshold SL for separating is set as shown in FIG. 2, a pulse signal supplied to a control circuit 10 for discrimination of television signal has a uniform pulse width and pulse interval as conjectured from FIG. 2, but the pulse width and interval of a pulse supplied to the control circuit 10 from the second sync separator 8 is ununiformed under the influence of the video signal. The control circuit 10 distinguishes whether the received signal is from the pay television broadcasting station or the normal one by checking the pulse width and interval of the pulse signal provided by the first and the second sync separators 7 and 8, and controls the switching circuit 9 through a control line 11. Then, the control circuit 10 provides the output signal from the invertor 3 as video signal at an output terminal 12 if the received signal is concluded that it is from the pay television broadcasting station, or, on the other hand, when it is judged to be from the normal one the output signal from the buffer 4 is supplied to the output terminal 12 as video signal.
In the prior art, since the discrimination is made as mentioned above and is made continuously, the control circuit 10 sometimes malfunctions due to noise or specified kind of video signal.